This invention relates to a switching regulator having improved characteristics for a television display arrangement.
In order to avoid the weight and cost of a line isolation transformer, television receivers may be supplied with power directly from the ac power line through a rectifier and filter. The filtered direct voltage will vary in proportion to the variations in the ac power line voltage, which may be undesirable. Also, the value of the filtered direct voltage will be approximately the peak value of the alternating-current input, and may be greater or less than the desired value.
It is possible to produce a regulated output voltage of lesser magnitude than the raw dc input by use of a series-pass regulator circuit, but this has the disadvantage of substantial power dissipation when the load current and/or the difference between the raw voltage and the regulated voltage are large.
Recently, an emphasis on reduced power consumption has lead to the increased use of switching regulators for powering television receivers. In switching regulators, a switch coupled to the raw direct-voltage supply is periodically turned on and off with a duty cycle adapted for regulation of the controlled voltage. U.S. Pat. No. 4,024,434 issued May 17, 1977 in the name of Joosten et al. illustrates the use of a transistor operated as a switch for reduced dissipation. While the power dissipation is reduced by this form of operation, transistors often have low gain and require considerable base drive in order to operate in the low-power saturated mode. Further, the inductor often associated with the switching regulator requires a so-called free-wheeling diode to prevent application of excessive voltages to the transistor at turn-off and to recover energy stored in the inductor.
By the use of controlled rectifiers such as SCRs the base drive problem associated with the use of transistor switches is obviated. The SCR is regenerative, and when gated into conduction remains conductive so long as forward bias is maintained across the main conduction path. Thus, a gate pulse may be momentarily applied to the SCR control electrode to initiate conduction and then need no longer be supplied to maintain conduction. The controlled-rectifier switch is turned off when forward current diminishes to zero and attempts to reverse, normally caused by application of a reverse voltage from an external source. The SCR is advantageous by comparison with the transistor not only because of its gating characteristics but also because application of reverse voltages exceeding the reverse voltage breakdown of the device does not result in its destruction but merely switches it into the conductive mode.
U.S. Pat. No. 3,970,780 issued July 20, 1976 in the name of Minoura describes a switching regulator in which an SCR is used as the control element for controllably charging a capacitor from the unregulated supply through the series connection of an inductor and a winding coupled to the horizontal deflection circuit. In the Minoura arrangement, the inductor must be small enough so that the current in the inductor and SCR can be reduced to zero during the retrace interval by the difference between the unregulated direct voltage and the turn-off voltage pulse across the winding. As a result, relatively large peak currents flow in the inductor and in the storage capacitor during the capacitor charging interval. These relatively large currents undesirably result in relatively large I.sup.2 R or heating losses. Also, the turn-off requirements and the relatively large changes in regulator current with changes in load current such as those resulting from kinescope beam current changes produces large changes in regulator peak currents. These changes in peak current passing through the regulator SCR and turn-off windings result in changes in the amount of energy coupled between the winding and the horizontal output transistor of the deflection circuit, and contribute to retrace time modulation and storage time modulation in the base of the output transistor as a function of beam current. The storage time modulation causes bending of vertical lines displayed on the raster. It is desirable to reduce the retrace time modulation and bends resulting from beam current changes, to reduce peak currents and heating losses and to reduce the load-dependent variation in the voltage pulse available to the turn-off regulator SCR in order that a larger filter inductor may be used.